According to local authorities, US Secretary of Defense Robert Gates arrived in Afghanistan late Monday for a surprise visit to evaluate international efforts against rising violence led by Taliban rebels allied with Al-Qaeda.

The "clear concern is that for two or three years there has been an increase of overall level of violence (in Afghanistan)," Gates told reporters during a short stopover in Djibouti en route to Kabul.

The defense secretary said he wanted an update on the security situation here, especially in the south where the Taliban are most active, and to assess if there was any spillover from extremist activity in Pakistan.

The two neighbors share a long and porous border across which Islamist rebels regularly cross to carry out attacks on the government and the 55,000 international troops supporting it.

Gates, who was last in Afghanistan in June, was due to meet Afghan President Hamid Karzai on Tuesday as well as with the commanders of the US-led coalition and separate NATO-led International Security Assistance Force (ISAF).

His visit will inform a meeting in Scotland later this month with some of the ISAF nations that have troops in the south where Taliban militants are most active.

This year has been the worst of the Taliban-led insurgency since the hardliners were removed from government in late 2001 in a US-led invasion launched after they did not surrender Al-Qaeda leaders for the 9/11 attacks.

Nearly 6,000 people have been killed -- most of them rebels -- and there have been around 140 suicide blasts, the worst killed nearly 80 people a month ago in the north -- which has seen less of the unrest than the south and east.